An image forming apparatus represented by a laser beam printer is known, wherein a light beam emitted from a light source is deflected and scanned in a main scanning direction by a deflecting and scanning unit, and is collected toward a drum (a photosensitive body) which has a face to be scanned, and a latent image is formed on a drum surface. In such an image forming apparatus, the latent image on the drum surface is transferred onto an intermediate transfer belt which is placed between the drum and a developing roller and an image which corresponds to the latent image is formed onto the intermediate transfer belt.
In the image which is formed onto the intermediate transfer belt, density fluctuations may occur in a main scanning direction and a sub-scanning direction, respectively. One possible cause of the density fluctuations is process gap (PG) fluctuations. First, the density fluctuations of the image in the main scanning direction are considered. As a factor for this, parallel characteristics of the drum (the photosensitive body) and the developing roller are possible. For example, when the mutual parallel characteristics of the drum and the developing roller are lost, variations occur in capabilities of developing onto the drum, possibly causing density fluctuations with respect to the main scanning direction. Here, the density fluctuations linearly change in the main scanning direction.
Next, the density fluctuations of the image in the sub-scanning direction are considered. One factor for this may be decentering of the drum. For example, when a slight movement of an axle of the drum occurs, positions at which a distance from a rotational axle of the drum to a surface differs occur, so that positions occur in which there is a difference in a gap between the drum and the developing roller. This difference in the gap becomes a developing variation, which would affect the image as the density fluctuations in the sub-scanning direction.
A different factor may be circularity of the drum. For example, assume that there is a second drum with low circularity relative to a first drum, which is circular. Then, with the second drum, at a time of rotation thereof, a difference occurs in a gap between the drum and the developing roller depending on a rotational angle, which may become a factor for fluctuations in developing. Due to the above-described factors, density fluctuations in the sub-scanning direction occur for an image formed on the drum surface. These density fluctuations become periodic, which occurs with a rotational period of the drum.
Factors for the density fluctuations include other factors such as potential variations of the drum, toner supply, toner removal, discharging, cleaning, etc., so that, combining them with density fluctuations due to process gap fluctuations, causes dynamic fluctuations to occur in both the main scanning direction and the sub-scanning direction.
In order to reduce such density fluctuations, for example, a light amount adjustment is performed in accordance with a transmitting characteristic of optics in the main scanning direction, for example. Moreover, for correcting in the sub-scanning direction, there is known a technique in which, for example, correction data are created in accordance with sensitivity variations of a photosensitive body to change a light amount in the sub-scanning direction, and a failure due to a phase offset of a rotational period of the photosensitive body and the correction data is avoided by an arithmetic calculation.